a) Field of the Invention
The invention is directed to a method and an arrangement for recording intensity patterns occurring on a contact surface by means of frustrated total reflection, particularly for low-distortion recording of relatively large-area fingerprints, handprints or footprints or other parts of the skin.
b) Description of the Related Art
In known devices for taking fingerprint images and handprint images which work by total internal reflection (TIR) on prisms, a perspective imaging of the contact surface or support surface in which the image of the impression is distorted trapezoidally and highly compressed in one direction requires correction in order to achieve the same resolution and the same imaging scale at all locations in the image for the imaging of a given rectangular support surface on a (usually square) area sensor.
A number of possibilities for correcting this distortion have been disclosed:
additional prisms for canceling the perspective distortion and adapting the imaging scale in horizontal and vertical direction (e.g., U.S. Pat. No. 5,650,842 or EP 0 308 162 A2),
telecentric imaging optics, also in combination with cylindrical lenses, for adapting the imaging scale in horizontal and vertical direction (e.g., U.S. Pat. No. 5,900,993),
holographic surfaces (e.g., U.S. Pat. No. 5,892,599),
cylindrical lens systems, although these are not suitable for forensic applications due to insufficient possibilities for correction (see, e.g., Stoltzmann et al., “Versatile anamorphic electronic fingerprinting: Design and manufacturing considerations”, SPIE, vol. 2537, Aug. 1995, 105–116),
rectifying multi-element structures (e.g., R. T. Hebert, “Off-axis optical elements in integrated, injection-molded assemblies”, SPIE, vol. 2600, December 1995, 129–134).
Devices with additional correction prisms require a rigid arrangement of the prisms and imaging optics elements relative to one another, which is disadvantageous particularly when installing the device within a predetermined volume. Further, the correction prisms are expensive to produce compared to round optics and have a relatively large mass which is disadvantageous for the total mass of the device. In addition, the chromatic aberration occurring in the arrangement limits the optical performance of a system of this kind. A description of the problems involved in the use of correction prisms is given by Stoltzmann et al. in their article “Versatile anamorphic electronic fingerprinting: Design and manufacturing considerations” (op.cit.).
Devices with telecentric imaging optics generally have low light power and therefore require a stronger illumination, high-sensitivity sensors or a long time for image generation and are therefore susceptible to interference during image acquisition. The different imaging scale in horizontal direction and vertical direction can be extensively corrected through the use of cylindrical lenses. However, like the large field lenses needed for telecentric imaging, cylindrical lenses are too expensive for applications with large recording areas.
In general, cylindrical optics are difficult to produce (and therefore expensive) and, above all, can not be corrected sufficiently to enable their use for forensic purposes.
The use of holographic surfaces as described, e.g., in “Prism fingerprint sensor that uses a holographic optical element”, Applied Optics, vol. 35, September 1996, 5242–5245, is also only suitable for use with small recording areas such as for recording individual fingerprints, since it is difficult to connect them to the prism without defects and they drive up the cost of the device.
Displacement optics and inclination optics (see, e.g., R. T. Hebert, op.cit.) which are shaped into wave-guiding multi-element structures require the production of complex optical surfaces which are difficult to produce and are therefore expensive. In this case also, large-diameter lenses are required in order to achieve an acceptable light intensity of the system, and it is not possible to eliminate distortion completely.